SelfLoader(3) SelfLoader(3)
NAME
SelfLoader - load functions only on demand
SYNOPSIS
package FOOBAR;
use SelfLoader;
... (initializing code)
__DATA__
sub {....
DESCRIPTION
This module tells its users that functions in the FOOBAR package are to
be autoloaded from after the __DATA__ token. See also the section on
Autoloading in the perlsub manpage.
The DATA token
The __DATA__ token tells the perl compiler that the perl code for
compilation is finished. Everything after the __DATA__ token is available
for reading via the filehandle FOOBAR::DATA, where FOOBAR is the name of
the current package when the __DATA__ token is reached. This works just
the same as __END__ does in package 'main', but for other modules data
after __END__ is not automatically retreivable , whereas data after
__DATA__ is. The __DATA__ token is not recognized in versions of perl
prior to 5.001m.
Note that it is possible to have __DATA__ tokens in the same package in
multiple files, and that the last __DATA__ token in a given package that
is encountered by the compiler is the one accessible by the filehandle.
This also applies to __END__ and main, i.e. if the 'main' program has an
__END__, but a module 'require'd (_not_ 'use'd) by that program has a
'package main;' declaration followed by an '__DATA__', then the DATA
filehandle is set to access the data after the __DATA__ in the module,
_not_ the data after the __END__ token in the 'main' program, since the
compiler encounters the 'require'd file later.
SelfLoader autoloading
The SelfLoader works by the user placing the __DATA__ token after perl
code which needs to be compiled and run at 'require' time, but before
subroutine declarations that can be loaded in later - usually because
they may never be called.
The SelfLoader will read from the FOOBAR::DATA filehandle to load in the
data after __DATA__, and load in any subroutine when it is called. The
costs are the one-time parsing of the data after __DATA__, and a load
delay for the _first_ call of any autoloaded function. The benefits
(hopefully) are a speeded up compilation phase, with no need to load
functions which are never used.
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SelfLoader(3) SelfLoader(3)
The SelfLoader will stop reading from __DATA__ if it encounters the
__END__ token - just as you would expect. If the __END__ token is
present, and is followed by the token DATA, then the SelfLoader leaves
the FOOBAR::DATA filehandle open on the line after that token.
The SelfLoader exports the AUTOLOAD subroutine to the package using the
SelfLoader, and this loads the called subroutine when it is first called.
There is no advantage to putting subroutines which will _always_ be
called after the __DATA__ token.
Autoloading and package lexicals
A 'my $pack_lexical' statement makes the variable $pack_lexical local
_only_ to the file up to the __DATA__ token. Subroutines declared
elsewhere _cannot_ see these types of variables, just as if you declared
subroutines in the package but in another file, they cannot see these
variables.
So specifically, autoloaded functions cannot see package lexicals (this
applies to both the SelfLoader and the Autoloader). The vars pragma
provides an alternative to defining package-level globals that will be
visible to autoloaded routines. See the documentation on vars in the
pragma section of the perlmod manpage.
SelfLoader and AutoLoader
The SelfLoader can replace the AutoLoader - just change 'use AutoLoader'
to 'use SelfLoader' (though note that the SelfLoader exports the AUTOLOAD
function - but if you have your own AUTOLOAD and are using the AutoLoader
too, you probably know what you're doing), and the __END__ token to
__DATA__. You will need perl version 5.001m or later to use this (version
5.001 with all patches up to patch m).
There is no need to inherit from the SelfLoader.
The SelfLoader works similarly to the AutoLoader, but picks up the subs
from after the __DATA__ instead of in the 'lib/auto' directory. There is
a maintainance gain in not needing to run AutoSplit on the module at
installation, and a runtime gain in not needing to keep opening and
closing files to load subs. There is a runtime loss in needing to parse
the code after the __DATA__. Details of the AutoLoader and another view
of these distinctions can be found in that module's documentation.
DATA, END, and the FOOBAR::DATA filehandle.
This section is only relevant if you want to use the FOOBAR::DATA
together with the SelfLoader.
Data after the __DATA__ token in a module is read using the FOOBAR::DATA
filehandle. __END__ can still be used to denote the end of the __DATA__
section if followed by the token DATA - this is supported by the
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SelfLoader(3) SelfLoader(3)
SelfLoader. The FOOBAR::DATA filehandle is left open if an __END__
followed by a DATA is found, with the filehandle positioned at the start
of the line after the __END__ token. If no __END__ token is present, or
an __END__ token with no DATA token on the same line, then the filehandle
is closed.
The SelfLoader reads from wherever the current position of the
FOOBAR::DATA filehandle is, until the EOF or __END__. This means that if
you want to use that filehandle (and ONLY if you want to), you should
either
1. Put all your subroutine declarations immediately after the __DATA__
token and put your own data after those declarations, using the __END__
token to mark the end of subroutine declarations. You must also ensure
that the SelfLoader reads first by calling 'SelfLoader->load_stubs();',
or by using a function which is selfloaded;
or
2. You should read the FOOBAR::DATA filehandle first, leaving the handle
open and positioned at the first line of subroutine declarations.
You could conceivably do both.
Classes and inherited methods.
For modules which are not classes, this section is not relevant. This
section is only relevant if you have methods which could be inherited.
A subroutine stub (or forward declaration) looks like
sub stub;
i.e. it is a subroutine declaration without the body of the subroutine.
For modules which are not classes, there is no real need for stubs as far
as autoloading is concerned.
For modules which ARE classes, and need to handle inherited methods,
stubs are needed to ensure that the method inheritance mechanism works
properly. You can load the stubs into the module at 'require' time, by
adding the statement 'SelfLoader->load_stubs();' to the module to do
this.
The alternative is to put the stubs in before the __DATA__ token BEFORE
releasing the module, and for this purpose the Devel::SelfStubber module
is available. However this does require the extra step of ensuring that
the stubs are in the module. If this is done I strongly recommend that
this is done BEFORE releasing the module - it should NOT be done at
install time in general.
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SelfLoader(3) SelfLoader(3)
Multiple packages and fully qualified subroutine names
Subroutines in multiple packages within the same file are supported - but
you should note that this requires exporting the SelfLoader::AUTOLOAD to
every package which requires it. This is done automatically by the
SelfLoader when it first loads the subs into the cache, but you should
really specify it in the initialization before the __DATA__ by putting a
'use SelfLoader' statement in each package.
Fully qualified subroutine names are also supported. For example,
__DATA__
sub foo::bar {23}
package baz;
sub dob {32}
will all be loaded correctly by the SelfLoader, and the SelfLoader will
ensure that the packages 'foo' and 'baz' correctly have the SelfLoader
AUTOLOAD method when the data after __DATA__ is first parsed.
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